The toxicity of 5-fluorouracil (5-FU) was decreased by two to eight-fold if this drug was injected in mice near the middle of the day (rest span) rather than in the middle of the night (activity span). If the rhythm in 5-FU toxicity is linked to the sleep-wakefulness endogenous circadian cycle across species, the least toxic time in man would correspond to 4.00 hours at night. The availability of a single-reservoir programmable-in-time external ambulatory pump (Chronopump, Autosyringe, Hooksett, USA) allowed us to perform a first test of this hypothesis. Five-FU was infused for 5 consecutive days, via an implanted venous access port, with peak drug delivery at 4.00 hours and no infusion from 18.00 to 22.00 hours. Each course was repeated after a free interval of 16 days. Intrapatient dose escalation was planned from 4 to 9 g/m2/course (800 to 1800 mg/m2/day x 5 days) if toxicity was less than grade 2 according to the World Health Organization (W.H.O.). Thirty-five patients with metastatic colorectal cancer were treated; 15 (41%) had received previous therapy, 22 (63%) had W.H.O. performance status of 2 or greater, and 19 (54%) had two or more sites involved. Grade 2 or greater toxicity was encountered in less than 5% of the courses, indicating an adequate control of toxicity via dose adjustment. Oral mucositis, diarrhea, and/or hand-foot syndrome limited dose escalation, and their incidence was dose dependent. Median maximal tolerated dose was 7.5 mg/m2/course in 30 patients assessed for this endpoint.(ABSTRACT TRUNCATED AT 250 WORDS)